Physical studies on the size and structure of the covalently closed circular chloroplast DNA from higher plants

The size and structure of the covalently closed circular chloroplast DNAs (ctDNA) from pea, lettuce, and spinach plants, have been studied by analytical ultracentrifugation. The values of so20,w,Na+ of the native and denatured forms of the open and closed circular DNAs from these plants have been determined. The absolute molecular weight of purified closed circular pea ctDNA monomers has been determined by buoyant equilibrium sedimentation to be 89.1 (S.D. +/- 0.7)-10(6). The value of the so20,w,Na+ of open circular pea ctDNA and its molecular weight, in conjunction with corresponding values for other sizes of circular DNA, has been used to derive an empirical relationship between so20,w,Na+ and molecular weight for open circular DNAs. Using this relationship, the molecular weights of lettuce and spinach ctDNAs have been determined to be 98.2 (S.D. +/- 1.5)-10(6) and 97.2 (S.D. +/- 1.5)-10(6), respectively. At pH values 12.7 and 13, closed circular lettuce and pea ctDNAs have been found to exist as mixtures of reversibly and irreversibly denatured closed circular DNAs.